Atari 520ST 4MB "agranlund" STRAM expansion - notes from an install

[oracle_jedi]

I bought an "agranlund" 4MB RAM expansion for my 520ST.

 

Its an open-source project that you can order up and make yourself.  I bought a ready-assembled one from an Ebay seller.  It seemed an easy way to add some functionality to the old 520ST.

 

You can download the project here: https://github.com/agranlund/STRam

The Youtube channel "Adrian's Digital Basement" featured the same expansion, although Adrian made the mistake of adding sockets to the install which meant it did not fit with the keyboard.

 

 

The install is not for the faint-hearted.  The instructions on the github page are scant.  Removing 16 RAM chips is not a trivial operation.  The 520ST motherboard is better made than much of the Tramiel era Atari PCBs, but it is still easily damaged.

 

Note: Leave the bypass caps in place.  These are still needed for the RAM expansion.

 

With the RAM chips removed and the expansion board installed, I followed Adrian's example and tied CAS1L, CAS1H and RAS1 to ground, and A9 to +5V.  This allowed the ST to boot up with 512K RAM.  Rather than run the lines under the board, I simply used pin 8 of U16 for ground, and pin 16 for +5V.

 

With the initial install done, next I had to connect the CAS1L, CAS1H and RAS1 lines.  Rather than solder lines directly to the MMU, I ran the lines to the pads on the top side of the motherboard using this guide from the Exxos forum: https://exxosforum.co.uk/atari/last/STM4MB/index.htm.   User Verault on the atari-forum.com reported that the CAS1L and CAS1H lines did NOT need to be swapped (https://www.atari-forum.com/viewtopic.php?t=43064), however on my install I absolutely did have to swap these.  Failing to do so resulted in a GEM desktop with corrupted menu items.  Note: I don't know if the silk screen on the expansion board is wrong, or the guide on exxosforum, but the picture is of my working install.

 

 

This boosted the ST to 1MB.

 

 

The A9 line needs to be run to MMU pin 64.  

 

 

I liked Adrian's idea of a switch to set the A9 to +5V which limits the ST to 1MB for compatibility with some software that objects to 4MB of RAM.  I placed a small switch next to the cartridge port which is held in place with hot-glue.   When reassembled, the switch can only be accessed with tweezers or a pen tip, but that's fine to me, as I do not want to accidentally knock it when the machine is running.

 

 

Cartridges can still be inserted with the switch in the 1MB position.  Switched to 4MB it can prevent some cartridges to be inserted, but I don't have many so that's not really a concern.  The +5V (force 1MB) is taken from the cartridge port.

 

 

With 4MB of RAM, booting from the Satan hard disk would be nice.  But TOS 1.0 does not seem to support this, so I replaced the ROM with TOS 1.4.  It took a couple of attempts to get the ROM chips in the right order:

 

 

With the case reassembled the ST would not even power up.   I eventually tracked this down to the top shield shorting on the RAM chips on the expansion board.  Since I like having the top shield in place, as it adds some support for the keyboard, I had to add a strip of Kapton tape to the underside of the shield.

 

 

The MMU had to be re-seated after the install, and all socketed ICs had to be firmly inserted several times before achieving a reliable boot.

 

Finally with everything reassembled, I can play Frontier off of a hard disk on the 520ST

 

 

I posted this here as I found the install guide frustratingly light on details.   I hope this might help others considering this upgrade.

 

[snarkdluG]

There are "fixed" agranlund pcb. That is why there are different instruction

[oracle_jedi]

6 hours ago, snarkdluG said:

There are "fixed" agranlund pcb. That is why there are different instruction

 

How can one tell which version of the PCB you have?  

 

The github repo still shows the need for a bodge wire.  If a revision was made to address a mislabelled CAS1L/CAS1H, then why not also fix the bodge wire issue?

 

I don't see any evidence in github of project revisions.   There looks to be just one with bare minimum instructions.

 

I do see a question from a user describing the similar corrupted menu items.  That user had to disable the RAS line to get a stable 2MB, so it sounds like perhaps the same issue?  The question is unanswered.

 

Had it not been for Adrian's Basement, I would have had no idea to try swapping CAS1L/CAS1H.  But the fact that another reported NOT having to do so makes me wonder if it is the ST Rev H motherboard that is mislabelled for the CAS1L/CAS1H tap points, and had I soldered directly to the MMU, I might not have had any such issue.

 

I do see that Exxos has a similar 4MB expansion for the 520ST, and I like that his version uses a more elegant PCB design that leaves access to most of the RAM chip locations, meaning that in the event of problems you have an easier task of visually verifying that the board is installed correctly, and there are not bridged solder joints.

 

[snarkdluG]

If I remember correctly, someone did do a fix and merged it on github with agranlunds github. I think he fixed the PCB. I can't find that thread at the moment.

 

I also did a PCB fix which is located here and on other forums. I never did github so it is sort of "hidden". It fixes the bodge wire need and confusion with CAS1L and CAS1H.

 

 

Edited January 28 by snarkdluG

[oracle_jedi]

7 hours ago, snarkdluG said:

If I remember correctly, someone did do a fix and merged it on github with agranlunds github. I think he fixed the PCB. I can't find that thread at the moment.

 

I also did a PCB fix which is located here and on other forums. I never did github so it is sort of "hidden". It fixes the bodge wire need and confusion with CAS1L and CAS1H.

 

 

Thanks for the links @snarkdluG  I answered the question posed on the github repo with a link to this thread.  Hopefully anyone looking over the project will find it, and your fixes, and save themselves some headaches.